The present invention relates to a non-linear resistance element having a large voltage dependence of the electric resistance or, more particularly, to a so-called varistor element of titanium dioxide-based sintered body and a method for manufacturing same.
Non-linear resistance elements, for example, a ceramic varistor exhibits non-linear relationship between the value of the electric current across a sintered ceramic body and the voltage applied between the electrodes provided on the surfaces of the ceramic body and, accordingly, the value of the electric resistance of the body is not a constant value but drastically decreases in the region where the voltage applied to the electrodes exceeds certain threshold value called a varistor voltage. Utilizing this unusual property in an electric circuit, varistors have found wide applications, for example, for the noise suppression of small-sized DC motors used in or relating to acoustic instruments, protection of contacting points of relays, discharge absorption in Braun tube circuits of color television sets and the like.
Conventional materials for the prior art ceramic-based varistor elements are, for example, tin oxide SnO.sub.2, iron oxide Fe.sub.2 O.sub.3, silicon carbide SiC and the like. Sintered bodies of tin oxide or iron oxide are themselves linear resistor elements and the non-linearity necessary for a varistor element is obtained by forming a potential barrier between the surface of the sintered body and a specific electrode provided on the surface of the body. The electrodes for silicon carbide-based varistors are not limitative since the non-linearity of the element is warranted by the interfacial phenomenon in the grain boundaries of the silicon carbide.
Conventional ceramic varistors of the prior art, however, suffer from several defects or problems such as the undesirably high production cost due to the difficulties in manufacturing the sintered bodies and forming electrodes, deterioration of the non-linearity in the lapse of time and the relatively high varistor voltage unsuitable for noise suppression in small-sized DC motors driven at low voltages.
An improvement to overcome the above described drawbacks in the ceramic varistors of prior art has been proposed recently, according to which the sintered ceramic body is shaped with titanium dioxide as the main ingredient admixed with very small or trace amounts of bismuth oxide and an oxide of a semi-conductive element such antimony, niobium, tantalum and the like (see, for example, Japanese Patent Publication 52-235 and U.S. Pat. No. 3,715,701). Sintered bodies of this type exhibit themselves a non-linearity in the voltage-current relationship with considerably low varistor voltage so that they are suitable for low-voltage uses such as the noise suppression in small DC motors in so far as the non-linearity of the sintered body is the only matter concerned.
One of the problems in the titanium dioxide-based ceramic varistors is, however, in the manner of contacting between the sintered body and the electrode. In the varistor elements of prior art, the contact between the sintered body of titanium dioxide and the electrode is non-ohmic so that a phenomenon of rectification appears in the interface between them overlapping the non-linearity in the voltage-current relationship of the sintered body itself. Therefore, the performance of the varistor element is made less clear-cut and the excellent non-linear characteristics of the sintered body cannot be fully utilized.